African trypanosomes (Trypanosoma brucei ssp.) are parasitic protozoa that cause human African trypanosomiasis (HAT, sleeping sickness), as well as nagana in cattle and other livestock. These diseases have devastating impact throughout sub-Saharan Africa where the tsetse fly vector is found. Some 60 million people in 36 countries are at risk of tsetse bite, and therefore of transmission, and as recently as 2000 the WHO estimated that there were 300,000 or more new infections per year. Only a handful of drugs are in use for treating HAT, the best of which (eflornithine) is expensive and requires a difficult regimen, the worst of which (melarsoprol) kills up to 10% of recipients. Infection is inevitably fatal without intervention, and since vaccination is not an option there is a critical need for new drug development. Toward this end a better understanding of the basic biology of the parasite is essential, particularly of processes that may be amenable to therapeutics. Such a process is the biogenesis of the parasite lysosome because it impacts the host- pathogen balance in multiple ways. Expression of lysosomal activities is differentially regulated through the trypanosome life cycle [1], and there are stage specific differences in the biosynthetic trafficking of essential lysosomal components [2]. The lysosome is the final repository of endocytic cargo acquired from host serum for nutritional purposes [3], as well as for potentially lytic immune complexes removed from the cell surface [4]. Release of the lysosomal protease TbCatL (trypanopain) is a factor in the signature event of human infection, penetration of the central nervous system [5]. Lysosomal physiology is critical to the activity of an innate human serum resistance trait, trypanolytic factor, which limits the host range of Trypanosoma species [6]. And finally, lysosomal hydrolytic activities have drawn considerable attention as potential chemotherapeutic targets [7]. Lysosomal biogenesis and function in trypanosomes are intimately related to the larger topic of secretory/endocytic trafficking. Much progress has been made in defining these processes, particularly regarding export and recycling of the bloodstream stage variant surface glycoprotein. However, little is known about pathways, either biosynthetic or endocytic, leading to the lysosome. In the previous funding period we initiated studies based on a single lysosomal membrane protein p67. In this proposal we intend to build on our previous studies of p67 (Aim #1), but also to extend our focus to other components of the lysosome (Aims #2 & 3). The purpose of this work is to enlarge our understanding of this critical aspect of basic trypanosome biology. It is our firm belief that this effort will help lay the foundation for future drug development, and will also illuminate not just the differences, but also the similarities of cell biological processes common the full range of eukaryotic evolution.